calc_example

Total head, N.P.S.H. and other calculations…14
3. Calculate the specific speed, suction specific speed and Thoma number and
check the prediction of the Thoma number regarding cavitation
Specific speed (N S )
Specific speed is a number that provides an indication of the speed of the impeller, the
flow rate and the head produced. The number is low, below 2000 (see Figure 13) for
pumps of radial design that provide high head and low flow. It is large, over 10000, for
pumps that provide high flow and low head. Along with the suction specific speed, it can
be used to predict cavitation.
N(
rpm)
× Q(
USgpm)
N S
=
0.75
H(
ft fluid)
[9]
N
S
1780 ×
=
97
500
=
0 .75
1287
[9a]
Predict the pump efficiency
The pump’s efficiency is directly related to its specific speed. Efficiency increases as
specific speed increases. Also, as shown in Figure 14, the efficiency increases as flow
rate increase, this means that larger pumps at the same specific speed are more
efficient. . For impeller sizes larger than 10” the effect of size or increased flow rate is
small and generally insignificant. For impeller sizes 4” and less , the penalty for smaller
sizes is severe.
The efficiency predicted by the chart in is Figure 14 is 0.77.
Suction specific speed (S)
Suction specific speed is a number that is dimensionally similar to the pump specific
speed and is used as a guide to prevent cavitation.
N(
rpm)
×
S =
N.
PS . . H.
A
Q(
USgpm)
( ft fluid)
0.75
[10]
Instead of using the total head of the pump H, the N.P.S.H. A (Net Positive Suction Head
available) is used. Also if the pump is a double suction pump then the flow value to be
used is one half the total pump output.
The Hydraulic Institute recommends that the suction specific speed be limited to 8500.
Some pump manufactures limit this value to 10,000-12,000.
S =
1780 ×
15.4
500
=
0 .75
5120
[10a]